The Islamic State posted a video on Tuesday that it said showed the beheading of James Foley, an American journalist who was kidnapped in Syria nearly two years ago, according to a transcript released by the SITE Intelligence Group.

The authenticity of the video, which was posted on YouTube, could not be immediately verified, and a telephone call placed to Foley's family was not immediately returned. YouTube later took down the four-minute, 40-second video.

Titled "A Message to America," the video shows the journalist kneeling in a deserted landscape, clad in an orange jumpsuit - an apparent reference to the uniforms worn by prisoners at the U.S. military detention camp in Guantanamo, Cuba. Standing to his left is a masked Islamic State fighter, who says that Foley's execution is in retaliation for the U.S. air strikes ordered by President Obama against the extremist group in Iraq.

Former hostages held alongside Foley in Syria - who were released earlier this year in exchange for multimillion-dollar ransoms - have said that before the air strikes began, the Islamic State had been asking for money in exchange for Foley's release.

"I call on my friends, family and loved ones to rise up against my real killers - the U.S. government - for what will happen to me is only a result of their complacent criminality," Foley says in the video, which was uploaded to the online account of the al-Furqan Media Foundation, according to SITE, which follows jihadist groups.

Foley, 40, a freelance journalist who was working for GlobalPost, an online publication based in Boston, as well as Agence France-Presse, went missing Nov. 22, 2012, in Syria. He was held alongside several other U.S. prisoners, whose families have insisted on a news blackout.

The video concludes with the fighter threatening to kill Steven Sotloff, another U.S. national who was being held alongside Foley. The young man is seen kneeling in the same position, in the same landscape and wearing the same, orange-colored jumpsuit. "The life of this American citizen, Obama, depends on your next decision," says the fighter.